1. Field of the Invention
The invention relates generally to the field of rotary tools used in drilling and completion of wellbores in the Earth. More specifically, the invention relates to rotary tools that make use of geared couplings between a driven input shaft and a driving output shaft.
2. Background Art
Drilling and completion of wellbores in the Earth, such as are used in the production of petroleum from subsurface reservoirs, includes the use of a number of types of rotary tools. A particularly important one of such rotary tools is the so-called “rotary steerable directional drilling system” (RSS). RSS systems are used in association with a drill string suspended from a drilling rig. The drill string is typically formed from lengths of steel pipe threaded together end to end and suspended from the rig by suitable hoisting equipment. Pumps force fluid called “drilling mud” through the interior of the drill string and out the bottom of the drill string through a drill bit, which performs the actual cutting of the rock formations. The exiting drilling mud cools the bit and lifts cuttings from the wellbore to the surface. The RSS system includes various steering or biasing devices thereon that selectively apply force normal to the axis of the drill string along a direction that the system operator intends to orient (“steer”) the trajectory of the wellbore. An advantage of RSS systems is that they enable continuous rotation of the drill string to turn the drill bit during drilling operations. Such continuous drill string rotation can provide increased drill rates as contrasted with other directional drilling devices, such as steerable motors, that require stopping drill string rotation to orient the device thus to steer the trajectory of the wellbore. A limitation to RSS systems known in the art is that increased rotational speed of the drill string can reduce the operating life and in-wellbore reliability of such systems. It is desirable, particularly with certain types of drill bits, to rotate the drill string at relatively high rates, thus making the use of such bits with RSS systems impractical.
Another application for using rotary motive power in a wellbore is submersible pumps. Submersible pumps are used to lift fluids from the wellbore to the Earth's surface when pressure in a subsurface reservoir is insufficient to move the fluids out of the wellbore unassisted. Electric submersible pumps include electrical wiring connected from an electric current source at the Earth's surface to an electric motor disposed in the wellbore and rotationally coupled to a pump, such as a positive displacement or centrifugal pump. One unavoidable aspect of using an electric motor in a wellbore to drive a pump is that it is necessary to seal the motor against intrusion of fluid from the wellbore. Such sealing requires sealing against the rotating motor shaft. The requirement to seal against the rotating shaft has made it impractical to use high speed motors in wellbores because high rotational speeds tend to limit seal life.
The above limitations could be addressed by using forms of reduction gear systems. Reduction gear systems when operated in wellbore have a number of limitations. First, it is necessary to provide a rotary seal somewhere in the system. The interior of as least part of the system is usually filled with a lubricant, such as oil, that is subject to degradation when exposed to high temperatures (typical in wells drilled into the Earth). The fluid in the interior of the system must ordinarily be pressure compensated to be maintained at the same fluid pressure as the hydrostatic fluid pressure in the wellbore, or the rotary seal will be subjected to differential fluid pressure in excess of its capacity to exclude wellbore fluid from the interior of the device. Pressure compensation devices known in the art may be subject to delays in compensation, causing fluid penetration into the interior of the device or fluid leakage. To limit fluid intrusion caused by such delay, preferably, the pressure compensation device maintains a slightly higher fluid pressure inside the device than in the wellbore. The slight pressure differential has the effect of causing slow, but constant loss of the pressure compensating fluid. Thus, even under ideal conditions the typical rotating seal device has a finite time that it can be used in a wellbore before removal to replenish the compensating fluid.
In the case of rotary devices using gears to multiply or reduce output speed relative to input speed, loss of lubrication can lead to gear failure. Application of abrupt high torque has also been known to cause gear failure. While the strength of the gears could otherwise be increased by increasing the size of the gears, such remedy is limited in the case of wellbore tools because such tools are typically limited in diameter to that of the wellbore being drilled less an annular space to allow cuttings and return mud flow to the Earth's surface. In wellbore drilling generally, such abrupt torque application is frequent, because of the highly variable mechanical properties of the Earth formations being drilled and the relatively low resolution control over the amount of axial force applied to the drill bit on the typical drilling rig.
Typical reduction gear devices used in wellbore tools include planetary gear sets. Planetary gears are particularly suitable for wellbore applications because in wellbore applications the input and output shafts of the gear devices are typically coaxial. Planetary gears are generally limited to about 3¼ to 3½ to 1 input to output ratio because of the limitations of shaft and gear diameters, among other factors. To step up or step down the speed between input and output shafts more than would be feasible with a single planetary gear set would require coupling a plurality of such gear sets end to end. Such arrangement increases the overall length, weight, complexity and required lubrication reservoir capacity of the gear set.
There exists a need to have a wellbore rotary device that can include a gear unit that does not require rotary seals or pressure compensation. There also exists a need for a rotary device for use in a wellbore that can have a relatively wide range of gear reduction ratios without the need for compound gear sets.